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    Subject[PATCH 10/37] x86/entry/32: Handle Entry from Kernel-Mode on Entry-Stack
    Date
    From: Joerg Roedel <jroedel@suse.de>

    It can happen that we enter the kernel from kernel-mode and
    on the entry-stack. The most common way this happens is when
    we get an exception while loading the user-space segment
    registers on the kernel-to-userspace exit path.

    The segment loading needs to be done after the entry-stack
    switch, because the stack-switch needs kernel %fs for
    per_cpu access.

    When this happens, we need to make sure that we leave the
    kernel with the entry-stack again, so that the interrupted
    code-path runs on the right stack when switching to the
    user-cr3.

    We do this by detecting this condition on kernel-entry by
    checking CS.RPL and %esp, and if it happens, we copy over
    the complete content of the entry stack to the task-stack.
    This needs to be done because once we enter the exception
    handlers we might be scheduled out or even migrated to a
    different CPU, so that we can't rely on the entry-stack
    contents. We also leave a marker in the stack-frame to
    detect this condition on the exit path.

    On the exit path the copy is reversed, we copy all of the
    remaining task-stack back to the entry-stack and switch
    to it.

    Signed-off-by: Joerg Roedel <jroedel@suse.de>
    ---
    arch/x86/entry/entry_32.S | 116 +++++++++++++++++++++++++++++++++++++++++++++-
    1 file changed, 115 insertions(+), 1 deletion(-)

    diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
    index e2621bf..b3477ff 100644
    --- a/arch/x86/entry/entry_32.S
    +++ b/arch/x86/entry/entry_32.S
    @@ -299,6 +299,9 @@
    * copied there. So allocate the stack-frame on the task-stack and
    * switch to it before we do any copying.
    */
    +
    +#define CS_FROM_ENTRY_STACK (1 << 31)
    +
    .macro SWITCH_TO_KERNEL_STACK

    ALTERNATIVE "", "jmp .Lend_\@", X86_FEATURE_XENPV
    @@ -320,6 +323,16 @@
    /* Load top of task-stack into %edi */
    movl TSS_entry_stack(%edi), %edi

    + /*
    + * Clear upper bits of the CS slot in pt_regs in case hardware
    + * didn't clear it for us
    + */
    + andl $(0x0000ffff), PT_CS(%esp)
    +
    + /* Special case - entry from kernel mode via entry stack */
    + testl $SEGMENT_RPL_MASK, PT_CS(%esp)
    + jz .Lentry_from_kernel_\@
    +
    /* Bytes to copy */
    movl $PTREGS_SIZE, %ecx

    @@ -333,8 +346,8 @@
    */
    addl $(4 * 4), %ecx

    -.Lcopy_pt_regs_\@:
    #endif
    +.Lcopy_pt_regs_\@:

    /* Allocate frame on task-stack */
    subl %ecx, %edi
    @@ -350,6 +363,56 @@
    cld
    rep movsl

    + jmp .Lend_\@
    +
    +.Lentry_from_kernel_\@:
    +
    + /*
    + * This handles the case when we enter the kernel from
    + * kernel-mode and %esp points to the entry-stack. When this
    + * happens we need to switch to the task-stack to run C code,
    + * but switch back to the entry-stack again when we approach
    + * iret and return to the interrupted code-path. This usually
    + * happens when we hit an exception while restoring user-space
    + * segment registers on the way back to user-space.
    + *
    + * When we switch to the task-stack here, we can't trust the
    + * contents of the entry-stack anymore, as the exception handler
    + * might be scheduled out or moved to another CPU. Therefore we
    + * copy the complete entry-stack to the task-stack and set a
    + * marker in the iret-frame (bit 31 of the CS dword) to detect
    + * what we've done on the iret path.
    + *
    + * On the iret path we copy everything back and switch to the
    + * entry-stack, so that the interrupted kernel code-path
    + * continues on the same stack it was interrupted with.
    + *
    + * Be aware that an NMI can happen anytime in this code.
    + *
    + * %esi: Entry-Stack pointer (same as %esp)
    + * %edi: Top of the task stack
    + */
    +
    + /* Calculate number of bytes on the entry stack in %ecx */
    + movl %esi, %ecx
    +
    + /* %ecx to the top of entry-stack */
    + andl $(MASK_entry_stack), %ecx
    + addl $(SIZEOF_entry_stack), %ecx
    +
    + /* Number of bytes on the entry stack to %ecx */
    + sub %esi, %ecx
    +
    + /* Mark stackframe as coming from entry stack */
    + orl $CS_FROM_ENTRY_STACK, PT_CS(%esp)
    +
    + /*
    + * %esi and %edi are unchanged, %ecx contains the number of
    + * bytes to copy. The code at .Lcopy_pt_regs_\@ will allocate
    + * the stack-frame on task-stack and copy everything over
    + */
    + jmp .Lcopy_pt_regs_\@
    +
    .Lend_\@:
    .endm

    @@ -408,6 +471,56 @@
    .endm

    /*
    + * This macro handles the case when we return to kernel-mode on the iret
    + * path and have to switch back to the entry stack.
    + *
    + * See the comments below the .Lentry_from_kernel_\@ label in the
    + * SWITCH_TO_KERNEL_STACK macro for more details.
    + */
    +.macro PARANOID_EXIT_TO_KERNEL_MODE
    +
    + /*
    + * Test if we entered the kernel with the entry-stack. Most
    + * likely we did not, because this code only runs on the
    + * return-to-kernel path.
    + */
    + testl $CS_FROM_ENTRY_STACK, PT_CS(%esp)
    + jz .Lend_\@
    +
    + /* Unlikely slow-path */
    +
    + /* Clear marker from stack-frame */
    + andl $(~CS_FROM_ENTRY_STACK), PT_CS(%esp)
    +
    + /* Copy the remaining task-stack contents to entry-stack */
    + movl %esp, %esi
    + movl PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %edi
    +
    + /* Bytes on the task-stack to ecx */
    + movl PER_CPU_VAR(cpu_current_top_of_stack), %ecx
    + subl %esi, %ecx
    +
    + /* Allocate stack-frame on entry-stack */
    + subl %ecx, %edi
    +
    + /*
    + * Save future stack-pointer, we must not switch until the
    + * copy is done, otherwise the NMI handler could destroy the
    + * contents of the task-stack we are about to copy.
    + */
    + movl %edi, %ebx
    +
    + /* Do the copy */
    + shrl $2, %ecx
    + cld
    + rep movsl
    +
    + /* Safe to switch to entry-stack now */
    + movl %ebx, %esp
    +
    +.Lend_\@:
    +.endm
    +/*
    * %eax: prev task
    * %edx: next task
    */
    @@ -769,6 +882,7 @@ restore_all:

    restore_all_kernel:
    TRACE_IRQS_IRET
    + PARANOID_EXIT_TO_KERNEL_MODE
    RESTORE_REGS 4
    jmp .Lirq_return

    --
    2.7.4
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